1. Field of the Invention
The present invention relates to a thermal overload trip apparatus which is applicable to an electrical device for protecting a motor and an electrical load device, such as a thermal overload relay or a manual motor starter, more particularly, to a thermal overload trip apparatus which is capable of efficiently adjusting a sensitivity thereof using an adjusting screw without adjusting an adjusting knob and a method for adjusting a trip sensitivity thereof.
2. Description of the Related Art
An overload protecting function, a basic function of a thermal overload trip apparatus, is implemented by performing a trip operation when an overload or overcurrent within a current range satisfying a pre-set condition for the trip operation is generated in an electric circuit. The current range may refer to a current range for the trip operation according to an IEC (International Electrotechnical Commission) standard specified as an international electrical standard. For example, a condition for the trip operation is that the trip operation should be performed within two hours when a current corresponding to 1.2 times of a rated current is conducted in a circuit and the trip operation should be performed more than two hours and within several hours when a current corresponding to 1.05 times of the rated current is conducted.
The thermal overload (overcurrent) trip apparatus generally includes a heater coil generating heat when an overcurrent is generated by being connected onto the circuit and positioned near the bimetals so as to provide a driving force for a trip operation by said bimetals being bent when the heater coil generates heat, so as to act as a driving actuator. One example of the thermal overload trip apparatus using the bimetals will be described with reference to FIGS. 1 and 2.
FIG. 1 is a diagram showing a configuration of a thermal overload trip apparatus in accordance with the related art, and FIG. 2 is a diagram showing a relation between an adjusting cam and a trip sensitivity adjusting range in the thermal overload trip apparatus in accordance with the related art.
In FIG. 1, a reference numeral 1 designates bimetals. Here, three bimetals are provided so as to be connected onto each circuit of three-phase Alternating Current. Thus, the bimetals are bent by heat from a heater coil (not shown) generating heat when an overcurrent is generated, and accordingly provide a driving force for a trip operation. A reference numeral 2 designates a shifter mechanism. The shifter mechanism 2 is a means for transferring the driving force for the trip operation from the bimetals 1 and is movable in a horizontal direction on the drawing by contacting the bimetals 1 in right and left directions so as to receive the driving force caused by the bent bimetals 1. In FIG. 1, a reference numeral 3 designates a trip mechanism. The trip mechanism is biased to be rotated in a direction of the trip operation by a spring (reference numeral not given). In FIG. 1, a reference numeral 4 designates a latch mechanism for releasing the trip mechanism 3 to be rotated in the direction of the trip operation or restricting the trip mechanism 3 not to be rotated in the direction of the trip operation. The latch mechanism 4 has one end portion installed to face a driving force transfer portion of the shifter mechanism 2 with each other so as to receive the driving force from the shifter mechanism 2, another end portion disposed on a rotation locus of the trip mechanism 3 so as to restrict or release the trip mechanism 3, and a middle portion therebetween supported by a rotation shaft (reference numeral not given) to be rotatable. A reference numeral 6 designates a contact point between the trip mechanism 3 and the latch mechanism 4 at the restriction position. In FIG. 1, at a position contacting one portion of the latch mechanism 4, an adjusting knob mechanism 5 is disposed to be rotatable so as to displace the latch mechanism 4 to be closer or to be distant to/from the shifter mechanism 2 resulting from changes of a contact pressure while contacting the latch mechanism 4. Here, the adjusting knob mechanism 5 includes a cam portion 9 having a varying radius of curvature of its outer circumstance, and an adjusting knob 10 coupled to the cam portion 9 or integrally extended from the cam portion 9 so as to rotate the cam portion 9. In FIG. 1, a reference character y indicates a bending displacement (bending amount) of the bimetals and indicates a pre-set displacement amount (distance) of the bending bimetals 1 when a pre-set overcurrent is conducted in the circuit. And, a reference numeral Δy indicates an allowance for trip operation and indicates a pre-set gap between the shifter mechanism 2 and the latch mechanism 4 when the shifter mechanism 2 is displaced by the pre-set bending amount y of the bimetals 1 caused by generation of the pre-set overcurrent. The allowance for trip operation is adjustable by the adjusting knob mechanism 5.
In the meantime, referring to FIG. 2, a configuration of the cam portion 9 included in the adjusting knob mechanism 5 in accordance with the related art will be described.
In FIG. 2, a reference character a indicates a cam adjustable range covering angles between a maximum trip operation insensitive adjusting position 12 and a maximum trip operation sensitive adjusting position 13. However, since a manufacturer of the thermal overload trip apparatus in the related art has adjusted an initial position of the cam portion 9 such as an initially-set position for the cam portion 11 by rotating the adjusting knob 10 of FIG. 1 during manufacturing, a range allowing a user to substantially adjust the rotation angle of the cam portion 9 is a substantially-adjustable range for the cam b. In FIG. 2, a reference character c indicates an initially-set adjusting range for the cam.
Operation of the thermal overload trip apparatus in accordance with the related art will be described.
First, the trip operation will be described. When the heater coil (not shown) generates heat by the overcurrent in the circuit, the bimetals 1 are bent and moved rightward on the drawing. Accordingly, the shifter mechanism 2 is moved rightward on FIG. 1, that is in a shifter mechanism operating direction 7 applied when the overcurrent is generated by a value obtained by adding the allowance for trip operation Δy to the bending amount y by the driving force of the bimetals 1 bent more than the value adding the allowance for trip operation Δy to the bending amount y, accordingly the latch mechanism 4 is pressed rightward and then rotated in a counterclockwise direction on the drawing. Then, the trip mechanism 3 restricted by the latch mechanism 4 is released and then rotated in the tripping direction, that is in the counterclockwise direction by an elastic force of a spring (reference numeral not given), and accordingly a succeeding switching mechanism (not shown) is operated into a trip (circuit-opening) position and then the circuit is tripped (broken), thereby protecting the circuit and a load device.
Next, a sensitivity adjusting operation for the trip operation will be described with reference to FIGS. 1 and 2.
Under a state that the initial position of the cam portion 9 is adjusted by a manufacturer such as the initially-set position for the cam portion 11 in FIG. 2, if the user rotates the cam portion 9 of FIG. 1 in the counterclockwise direction, the latch mechanism 4 is rotated clockwise, centering the rotation shaft (reference numeral not given), that is in a trip operation sensitivity sensitive adjusting direction 8, accordingly the allowance for trip operation Δy becomes narrow and the trip operation sensitivity of the device with respect to the overcurrent becomes more sensitive.
Since the thermal overload trip apparatus in accordance with the related art has a configuration that the trip operation sensitivity is adjusted only by the cam portion and the latch mechanism, it is difficult to precisely specify relative positions between the cam portion and the latch mechanism and a driving force transfer structure thereof and relative positions between the latch mechanism and the shifter mechanism and a driving force transfer structure thereof and to install the apparatus based on a standard. Thus, the thermal overload trip apparatus in accordance with the related art has a possibility to cause defects during manufacturing that there is no allowance for trip operation or the tripping operation is not performed even if the cam portion is rotated to the maximum sensitive position.
And, since the thermal overload trip apparatus in accordance with the related art has a structure requiring disassembling and re-adjusting the relative positions between the components and the driving force transfer structure thereof when the defectiveness occurs in the manufacturing processes, it may deteriorate productivity of manufacturing.